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Old 22nd May 2010, 11:31 am   #1
'LIVEWIRE?'
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Default A puzzling PSU Fault!

The PSU in question is used with a 1969 Grundig Satelite multiband radio, which, apart from noisy switches, and a stiff tuning gang, is otherwise OK.
One diode in the B40C600 Bridge rectifier was o/c, so I've replaced that with a B40C1500 I happened to have, and also replaced the two Electrolytic Capacitors(2200uf main smoother, and 470uf across the zener diode in the conventional(for the time) circuit, using an AD162 series pass transistor.
The puzzling thing is that the mains transformer had overheated(so now I've to find a replacement)yet the PSU, if fed from 13 to 16v DC, produces a stable 6 or 9 volt o/p, so what caused the transformer to fail?? The primary is protected by a 50mAT fuse in each leg, neither of which blew, nor did the 630mAT fuse in the DC Negative o/p from the rectifier. The PSU was in the condition described when I received it. How could one o/c rectifier diode be responsible for this??
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Old 22nd May 2010, 11:53 am   #2
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Default Re: A puzzling PSU Fault!

If one diode in the bridge has gone O/C that would convert the rectifier from full wave to half wave. Apart from increased ripple current in the electrolytics there would be dc passing through the transformer. The ripple voltage across the main electrolytic would also increase and be at 50Hz instead of 100Hz. If the transformer is small, as I suspect the manufacturers would fit the smallest transformer they could as it would cost less, it would not be able to withstand the dc and would overheat and over time would gradually self destruct. As there is a series pass transistor and the dc output is significantly lower than the input voltage the output would not be significantly affected by the increased ripple voltage. To the user there would be no observable difference so they would continue to use it oblivious to the damage being caused to the PSU.

Well that's my theory.

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Old 22nd May 2010, 2:13 pm   #3
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Default Re: A puzzling PSU Fault!

Quote:
Originally Posted by KeithsTV View Post
Apart from increased ripple current in the electrolytics there would be dc passing through the transformer.
Which is likely to saturate the core, causing an increase in primary current. Sounds a likely explanation.
Rob.
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Old 22nd May 2010, 2:37 pm   #4
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Default Re: A puzzling PSU Fault!

Yes, I think that is about right. If the current pulse was the same width, then the transformer heating would double, as you get twice the current for half the overall time and heating goes like I squared times t. In reality the pulse will widen, so it won't be quite as bad as twice the heating. Still enough to overheat a transformer which was probably only just good enough before the fault.
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Old 22nd May 2010, 3:18 pm   #5
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Default Re: A puzzling PSU Fault!

This is common, though less so nowadays. Either Les Lawry-Johns or Eugene Trundle covered it in one of their books.
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Old 22nd May 2010, 7:29 pm   #6
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Default Re: A puzzling PSU Fault!

Thanks for the detailed explanation, Keith, and for your comments too, Dave & Keith. I guess I just didn't take in what the failure of one diode out of four would mean. I was taught all the theory nearly 40 years ago, but maybe my power of recall isn't what it once was! Anyway, one I fit a replacement transformer all should be OK. (It would probably be simpler to replace the old power supply with a modern 'wall wart', but the original is built into a box which fits in a dedicated space in the radio, connecting via PP9 size terminals on the PCB, and I would like to keep it that way if possible
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